Today monks are known for turning the other cheek, honoring saints, and blessing humanity with brotherly love. But for centuries they were known equally for fulminating their foes, humiliating saints, and casting calamitous curses at persons who crossed them. Clerics called these curses “maledictions.” This article argues that medieval communities of monks and canons used maledictions to protect their property against predators where government and physical self-help were unavailable to them. To explain how they did this I develop a theory of cursing with rational agents. I show that curses capable of improving property protection when cursors and their targets are rational must satisfy three conditions. They must be grounded in targets’ existing beliefs, monopolized by cursors, and unfalsiﬁable. Malediction satisﬁed these conditions, making it an effective institutional substitute for conventional institutions of clerical property protection.